Born in St Louis, Missouri, Hearn studied philosophy at Southwestern University before he embarked on a career in the theater, training for the stage with legendary acting coach Irene Dailey. Most of Hearn's early performances were in traditional productions at the New York Shakespeare Festival and theaters at Lincoln Center. He first garnered a bit of notice as John Dickinson in the acclaimed 1969 award-winning musical "1776" and as Liv Ullmann's leading man in "I Remember Mama" (1979). Ironically, Hearn was to grab headlines later that year, when he replaced Len Cariou in the title role of Stephen Sondheim's dark musical "Sweeney Todd" on Broadway, opposite Dorothy Louden as a woman who bakes his character's murder victims into pies. Hearn and original star Angela Lansbury later headed the show's touring company, then reprised their roles for a Showtime production of the musical, which brought him an Emmy for his chilling portrayal of the demon barber of Fleet Street. Hearn and Lansbury remained friends and the actress invited him to guest star on several episodes of her CBS sleuth series "Murder, She Wrote" in the early 1990s.
Although Hearn followed up "Sweeney Todd" with Tony-nominated performances in "Watch on the Rhine" (1980) and "A Doll's Life" (1983), it wasn't until he played flamboyant drag queen Albin in "La Cage aux Folles" (1984) that he got to actually take home one of the coveted prizes. The actor earned his second Tony when he played the creepy butler Max von Mayerling in "Sunset Boulevard". opposite Glenn Close and was nominated for a fifth time for his performance in the 1999 Sondheim revue "Putting It Together", which co-starred Carol Burnett, John Barrowman, Bronson Pinchot and Ruthie Henshell. He also reunited with Close for the TV-movie "Sarah Plain and Tall: Winter's End" that year.